In Nassau, heroin killed 62 people last year, while oxycodone was responsible for 47 fatalities and fentanyl another 20 — the most deaths ascribed to all three of those drugs in the county since modern record keeping began.

Overall, opiates killed at least 232 people in Suffolk and 210 in Nassau last year, according to the latest data compiled by the county medical examiner’s offices and analyzed by Newsday.

Experts said the most troubling aspect of the islandwide data is the increase in deaths caused by fentanyl and its derivatives — large amounts of which are being produced in overseas labs, then being smuggled into the U.S., authorities said.

“The heroin supply is getting more dangerous” because fentanyl is being added to the drugs, said Dr. Andrew Kolodny, co-founder of advocacy group Physicians for Responsible Opioid Prescribing, and a senior scientist at the Heller School for Social Policy and Management at Brandeis University. “It’s become increasingly difficult to find heroin that doesn’t have fentanyl in it.”

Fentanyl is at least 25 to 40 times more powerful than heroin. It is sometimes used to induce anesthesia before medical procedures, and even a small dose can be fatal.

Traffickers are increasingly adding the drug to their heroin supplies to give it a more powerful punch, officials said. In some cases, they’re even selling pure fentanyl to customers who believe they’re buying heroin, officials said.

Fentanyl and its chemical derivatives have been linked to the deaths of more than 700 people across the United States between late 2013 and early 2015, according to the Drug Enforcement Administration.

Law enforcement officials said the illicit mixtures are probably being made by chemists in China and Mexico, where the ingredients needed to produce them are more readily available. It’s not clear at what point in the supply chain the chemicals are being added to local heroin supplies.

Heroin continued to cause the most deaths among all opiates in Nassau and Suffolk in 2015, continuing a yearslong trend. Suffolk tallied more fatal heroin overdoses than any county in New York from 2009 to 2013, including each of the five boroughs, records show.

At least 337 people died from heroin in Suffolk during the 60-month period ending Dec. 31, 2013, data shows. That’s more than in the Bronx, which recorded 216 such deaths, the second-highest number over that period; Queens, which recorded 174; Manhattan, which recorded 156; Brooklyn, with 151; and Nassau, with 128.

“Our kids are dying,” said Suffolk Police Commissioner Timothy Sini, who has instituted a host of new anti-drug programs in the county this year, including the creation of a citizen tip line. “We’re going to do everything we can to fight this scourge.”

A recent Nassau County report on opiates, obtained exclusively by Newsday, said that “opioid intoxication remains a major health and safety concern within Nassau County,” adding that testing for the drugs has “disproportionately challenged the available resources of the Medical Examiner’s Toxicology Laboratory.”

County officials say they are working to save as many lives as possible.

“Nassau’s awareness, education, enforcement and treatment initiatives are making progress inasmuch as far fewer overdoses are occurring in Nassau County than neighboring municipalities,” said Nassau County Executive Edward Mangano, a reference to the fact that fewer people died from opioids in Nassau last year than in Suffolk or New York City.

Fatal overdoses caused by opiates have risen dramatically across the United States in recent years, driven by a surge in prescription pill addiction and a flood of cheap heroin from Mexico, local law enforcement officials say.

In 2014, opioids were linked to a record 28,647 deaths in the United States, government data show. Since 2000, the number of confirmed fatal opioid overdoses in America has quadrupled.